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Physics of the Eye

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Physics of the Eye

The eye is the organ responsible for sight. It converts light into electrical signals and sends them to the brain. As mammals, our eyes are placed at the front of the head to allow binocular vision; this gives us depth of vision. This section will delve into the mechanism of light perception and the Physics of the eye.

Physics of the eye: Vision

The eye is a spherical structure. It can perceive images thanks to the refraction that occurs within the inhomogeneous liquid that fills it. This liquid is called vitreous humor. The light arrives at the cornea, the most external part of the eye, and this focuses it towards the pupil, whose aperture is regulated by the iris. The iris is also the part that gives our eyes their color. The stroma can be from light brown to black and even green, blue or hazel, thanks to the phenomenon of light scattering on this surface.

Behind the pupil is the lens that projects the light to the retina on the opposite side of the eye. The optic nerve there captures the image, transforms it into an electrical signal and sends it to the brain.

Physics of the Eye, Eye structure, StudySmarterFigure 1.- Structure of the eye. Source: Wikipedia (CC by 3.0)

There are many types of eyes, subdivided into two categories, compound and non-compound. These have further subcategories:

  • Non-compound eyes, characterized by having pit, spherical lenses, multiple lenses, refractive cornea, reflector eyes.
  • Compound eyes. Typical of arthropods like insects and crustaceans. Mantis, flies, lobsters, bees and so on have this kind of eye structure. characterized by having apposition, superposition, parabolic superposition, others.

Physics of the Eye, Detailed eye of a fly, StudySmarterFigure 2.- Detail of the compound eye of a fly. Source: USGS, public domain.

Physics of the eye: The eye as a lens

To understand the main mechanism that makes the eye work, we can examine how a lens works and what it is. A lens is a transparent interface that allows visible light to pass through, ideally without any reflections (loss in the incident energy).

Not all lenses are transparent, some are semi-transparent, and others are opaque. A lens can be concave or convex.

Physics of the Eye, Light rays through a lens, StudySmarterFigure .3 - light rays (in blue) passing through a lens. The light is focused at a point called focal point, in red. In the eye, the same mechanism occurs, and the focal point corresponds to the retina, Toma - StudySmarter Originals

Physics of the eye: Light and color processing

Visible light has two qualitative aspects, intensity and color. Inside the retina, there are photosensitive cells called rod cells and cone cells or just cones. Cone cells are situated in the center of the retina and have less sensitivity to light than rod cells.

There are three types of cone cells in the eye, and each type can catch a different range in the light's spectrum. L-cones are those associated with lower wavelengths; hence the name, particularly with the color blue; M-cones are associated with green and S-cones with red. M and S stand for middle and small, respectively, referring to the wavelength they can sense.

Rod cells are on the outer edges of the retina. There are twenty times more rods than cones, and their sensitivity is a five hundred to a thousand times more than that of the cones. They are responsible for night vision and peripheral vision, but they are not involved in the perception of colors. This explains not only our trichromatic vision but also why we cannot distinguish colors at night.

Physics of the Eye, Spectrum of trichromatic vision, StudySmarterFigure 4.- Spectrum of trichromatic vision, the peaks correspond to the maximum values o sensitivity o the cone cell in the human eye. Source BenRG, public domain.

Physics of the eye: Defects of vision

The eye is a sphere to catch the light from a wide angle, but a spherical lens will alter an image (Figure 5). The inhomogeneous liquid corrects this inside the eye because it has a refractive index higher than the air. This avoids image alterations.

Physics of the Eye, Light passing through a water drop, StudySmarterFigure 5.- A drop of water alter light after passing through it. Source: Nan Fry (CC by-SA 2.0 ).

Sometimes floaters form in this liquid, little clumps of viscous material inside the eye. They often appear within the range of vision but are harmless.

Alterations of the structure can cause defects in vision. For example, myopia occurs when the focus of the eye's lens is in front of the retina instead of on it. These distant objects to be blurry causes. When the focus of the lens is behind the retina, we talk of hypermetropia.

How can we correct defects of vision?

Many things can go wrong with vision. Corrections range from using glasses to surgical operations depending on which part of the eye is compromised or not working properly.

Physics of the eye: Glasses and lenses

When the problem is an alteration of focus within the eye, it is possible to correct it with appropriate lenses. This works by modifying the angle of incidence of the light to correct the focus. For instance, a person with myopia needs lenses that change the angle of the light to focus further away. If this is combined with the shortened focus of the eye, the result is the correct alignment of the image on the retina.

Physics of the Eye, Hypermetropia correction diagram, StudySmarterFigure.6 - Correction of hypermetropia. The lens alters the incoming light in a way that the eye's lens focuses on the retina, Camacho - StudySmarter Originals

Physics of the Eye (A2 Only) - Key takeaways

  • Eyes are structures that allow us to perceive images. Eyes focus the light that approaches the cornea to reach a photosensible part of the eye called the retina.
  • The retina has two kinds of cells called cones and rods. These are responsible respectively for color vision and night vision. Cones are specialized in detecting colors but are less sensitive to light, and rods are more sensitive to light but unable to distinguish between colors.
  • The eye is spherical, and this allows a wide range of sight. It also has an inhomogeneous liquid inside it, which makes corrections using refraction.
  • It is possible to use lenses to correct focus-related sight problems. These modify the light approaching the eye in order to achieve the correct incidence on the retina.

Frequently Asked Questions about Physics of the Eye

The stroma of the iris is the part that gives color to the eye. It varies from light brown to black and, in the case of blue, green or hazel eyes, is due to the scattering of light on the stroma.

There are two main categories of eyes: compound and non-compound, and each of them has several subcategories.

These are known as floaters. Little clumps of vitreous material can form in the liquid present in the eye, casting shadows on the retina.

Final Physics of the Eye Quiz

Question

What's the retina?

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Answer

The part of the eye where the image is focused.

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How does the iris work?

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Answer

It regulates the amount of incoming light.

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Can we see colors in the dark?


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No, because the cells operating in the night don't distinguish colors.

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What's myopia?

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A defect in the focus inside the eye

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What allows us to see colors?

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We have specialized cells that catch a range of colors.

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What's the cornea?

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The most external part of the eye.

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What's the difference between the iris and the pupil?

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The iris is responsible for the aperture of the pupil.

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What is the shape of the eye, and why does it have liquid inside?

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The eye is spherical, and the liquid inside causes the light to refract on the retina.

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What are cone cells?

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Answer

Cone cells are used to see colors and are highly sensitive to light.

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Why rod cells aren't involved in color vision?

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Because rod cells are of one kind.

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How can vision defects occur?

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When the eye's structure is altered, misalignments of the focus can occur, leading to vision defects.

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What do we do to correct focus-related vision problems?

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By applying lenses in the front of the eye, we correct the angle of incidence of the light, allowing it to reach the correct point in the retina.

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What's hypermetropia?

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It is the condition that causes the focus of the eye's lens to be in front of the retina.

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How many types of cone cells exist?

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Three, they perceive light of different wavelengths.

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Are cones useful for night vision?

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Yes, because they are more sensitive to light than the rod cells.

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What are defects of vision?

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Defects that prohibit clear vision due to the physical structure of the eye.

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Which of the following is not a common vision defect?

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Answer

Amblyopia.

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What is myopia?

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It is a defect that causes distant objects to seem blurry.

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What is a cataract?

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It is when a membrane forms in front of the eye lens due to age.

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What is hypermetropia?

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It is a defect that causes near objects to seem blurry.

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What is astigmatism?

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It is a defect causing parts of an image to seem sharper or duller than others.

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How is myopia treated?

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Using concave lenses.

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How is hypermetropia treated?

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Using convex lenses.

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Which of the following is the difference between concave and convex lenses?

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Concave lenses have a negative focal length, while convex lenses have a positive one.

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A person reading a book cannot see clearly when holding the book at a distance of less than 25 cm from the eye. What type of vision defect do they have?

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Answer

Hypermetropia.

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Mary cannot hold her book closer than 90 cm. The near point of a normal eye is 25 cm. Prescribe a corrective lens of suitable power to correct her vision.

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3D.

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What is laser surgery, and when is it used?

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It is a corrective measure for defect of vision that corrects the shape of the retina in case of astigmatism or other defects.

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The far point of the eye is 120 cm away from the eye. Determine the focal length and power to correct its vision so that distant objects can be seen clearly.

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Answer

-0.83D.

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Someone has difficulty seeing distant objects and reading a book that is closer than 25 cm from their eyes. What is their condition, and what correction method is required?

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Their condition is both presbyopia and myopia, which can be corrected with bifocal lenses.

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What is needed for the creation of an image?

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Both are correct answers.

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What controls the amount of light entering the eye?

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Answer

Pupil.

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Which eye part regulates the radius of the pupil?

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Iris.

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What is the cornea?

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It is a transparent membrane covering the front portion of the eye.

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By what range does the pupil contract and expand?

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The pupil expands to around 8mm in low light and contracts to 1.5mm in strong light.

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What is a photopigment?

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A photopigment is a substance found in light-sensitive cells known as cone cells.

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What are the two types of light-sensitive cells?

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Cone cells and rod cells.

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What are rod cells?


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Answer

Light-sensitive cells that allow us to see at night.

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What are cone cells?

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Cones are the cells that allow us to see in colour.

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What is the spectrum range in nanometers of the light we can perceive?

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Answer

380nm to 750nm

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What is the peak sensitivity of rod cells?

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500nm.

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Question

The ciliary muscle contracts when we focus on an image. True or false?

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Answer

True. It does so to refract the light and change the focus.

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The classification of cone cells is L-cones, S-cones, and M-cones. True or false?

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Answer

True. Cone cells can be divided into L-cones (sensitive to red and yellow), M-cones (sensitive to green), and S-cones (sensitive to blue).

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Do cone cells react to the different frequencies of light?


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Answer

Yes.

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Question

The light we observe comes from two sources. Choose the correct option below.

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Answer

Emitted and reflected light from bodies.

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What is a lens?

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Answer

A lens is a transmissive optical device that employs refraction to concentrate or scatter light.

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What are some applications of a lens?

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Answer

A lens has various applications in imaging devices such as telescopes, binoculars, and cameras.

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What is a converging lens?

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Answer

A lens is known as a converging lens because, when parallel rays are incident on it, the emerging rays converge at a point.

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What is a diverging lens?

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In a diverging lens, when parallel rays are incident on it, the emerging rays spread out or diverge.

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What is a principal axis?

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Answer

The principal axis is a horizontal line that passes through the lens’s centre and is perpendicular to the lens.

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What controls lens focal length?

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Answer

Its shape.

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